The present invention relates in general to drive systems for electric vehicles, and, more specifically, to the rapid discharging of capacitors when shutting down the electric drive system.
Electric vehicles, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), use inverter-driven electric machines to provide traction torque and regenerative braking torque. Such inverters typically employ an energy storage capacitor (or the main capacitor) as the DC link, which is usually interfaced with the high-voltage (HV) battery through a variable voltage converter (VVC), an input capacitor, and a pair of mechanical contactors (collectively forming a drive system).
A shutdown of the drive system can result from a vehicle key-off, a high-voltage DC interlock fault, or a vehicle crash, for example. During shutdown, the HV battery is quickly isolated from the rest of the electric system by opening the mechanical contractors. However, there will still be HV electric charge on the input capacitor and the main capacitor. Due to safety requirements, those HV electric charges should be quickly discharged within a specific time.
One conventional discharging method operates as follows. Once the open state of the contactor is confirmed, upper and lower switches (e.g., IGBTs) of the VVC are disabled. The inverter switches the main capacitor voltage into the machine load (which may be the motor or the generator) in order to dissipate the electric charge on the main capacitor by pushing a calibratable flux-weakening current into the motor and/or the generator. The flux-weakening current includes a negative D-axis current component and a zero Q-axis current component which is preferably controlled to produce zero torque in the machine.
As a result of the current flowing through the inverter to the machine, the energy stored in the main capacitor is converted into losses in the machine windings and the IGBT switches. Consequently, the voltage on the main capacitor starts to drop. Once the voltage on the main capacitor drops below that of the input capacitor, the reverse-blocking diode of the upper-leg IGBT in the VVC begins to conduct, forcing the voltage on both capacitors to be approximately the same. The flux-weakening current begins to discharge both the main capacitor and the input capacitor simultaneously. Once the main capacitor voltage and the input capacitor voltage drop below a voltage threshold, the main capacitor voltage can be maintained at this level through active bus voltage regulation—which may be desirable when the shutdown occurs while the motor or generator are rotating in order to allow a controlled ramping down of the rotation. Then once the motor and generator speed drops below a speed threshold, the inverter continues to operate so that the discharge current ramps down to zero, whereupon the inverter IGBTs are turned off and the discharge is completed.
Although the conventional method works adequately in many circumstances, it has some limitations. Ideally, if the motor/generator inverters are pushing flux-weakening currents into the motor and the generator, the voltage on the main capacitor should go down. Because of possible inaccuracies in the position signals for the machines, however, a negative Q-axis current may be injected into the machines so that a regenerative torque may be produced. In such cases, the voltage on the main capacitor may go up instead of going down. This effect is more likely to happen when the motor or generator speed is high. Thus, the robustness of the conventional discharge strategy against position sensor inaccuracy may be less than desired.
In the conventional method, the discharge of the input capacitor stops when the voltage reaches a predetermined level. The value for this level is typically low enough to ensure safety to human beings, but it must also be chosen to be high enough to maintain stable current control of the permanent magnet machines within a certain speed range (which is necessary due to the position sensor inaccuracy noted above). If the motor and generator speed is above this speed range, stable current control may not be available at a safe level of the DC bus voltage. Thus, the operating conditions where the conventional discharge strategy is fully functional are limited.
In addition, it would be desirable to decrease the time required to discharge the capacitors. Typically the main capacitor has a much larger capacitance than the input capacitor and usually operates with a much higher voltage. Discharging the main capacitor first to the voltage level of the input capacitor and then further discharging the two capacitors together significantly limits the discharge speed for the input capacitor, even though the reduction of the input capacitor voltage may be more important for human safety concerns.